Computer Networks 56 (2012) 642–660 Contents lists available at SciVerse ScienceDirect Computer Networks journal homepage: www.elsevier.com/locate/comnet Adaptive Search Radius – Using hop count to reduce P2P traffic q ⇑ Ricardo Lopes Pereira a, , Teresa Vazão a, Rodrigo Rodrigues b a INESC-ID/Instituto Superior Técnico, Technical University of Lisbon, Av. Professor Cavaco Silva, 2744-016 Porto Salvo, Portugal b Max Planck Institute for Software Systems (MPI-SWS), Kaiserslautern and Saarbrücken, Germany article info abstract Article history: Peer-to-Peer (P2P) file sharing accounts for a very significant part of the Internet’s traffic, Received 14 May 2010 affecting the performance of other applications and translating into significant peering Received in revised form 8 April 2011 costs for ISPs. It has been noticed that, just like WWW traffic, P2P file sharing traffic shows Accepted 24 October 2011 locality properties, which are not exploited by current P2P file sharing protocols. Available online 31 October 2011 We propose a peer selection algorithm, Adaptive Search Radius (ASR), where peers exploit locality by only downloading from those other peers which are nearest (in network Keywords: hops). ASR ensures swarm robustness by dynamically adapting the distance according to Peer-to-Peer file part availability. ASR aims at reducing the Internet’s P2P file sharing traffic, while File sharing Traffic reduction decreasing the download times perceived by users, providing them with an incentive to Performance improvement adopt this algorithm. We believe ASR to be the first locality-aware P2P file sharing system Locality aware that does not require assistance from ISPs or third parties nor modification to the server infrastructure. We support our proposal with extensive simulation studies, using the eDonkey/eMule protocol on SSFNet. These show a 19 to 29% decrease in download time and a 27 to 70% reduction in the traffic carried by tier-1 ISPs. ASR is also compared (favourably) with Biased Neighbour Selection (BNS), and traffic shaping. We conclude that ASR and BNS are comple- mentary solutions which provide the highest performance when combined. We evaluated the impact of P2P file sharing traffic on HTTP traffic, showing the benefits on HTTP perfor- mance of reducing P2P traffic. A plan for introducing ASR into eMule clients is also discussed. This will allow a progres- sive migration to ASR enabled versions of eMule client software. ASR was also successfully used to download from live Internet swarms, providing signif- icant traffic savings while finishing downloads faster. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction distribute large files due to its inherent scalability: the more peers join the network, the more bandwidth is Over recent years Peer-to-Peer (P2P) File Sharing (FS) available. applications have become very popular. They are used all P2P networks allow software producers and indepen- over the world as an effective way of distributing files to dent artists to distribute content without the need for a large population [1]. P2P is particularly well suited to the large investments in servers and network connectivity required by conventional approaches. Established content producers themselves have begun to warm up to the idea q This work was partially supported by FCT (INESC-ID multiannual of using P2P technology as a sales channel, instead of funding) through the PIDDAC Program funds. regarding it solely as a threat [2]. The commercial endorse- ⇑ Corresponding author. ment of P2P FS networks will allow them to continue their E-mail addresses: [email protected] (R. Lopes Pereira), [email protected] (T. Vazão), [email protected] growth as the dominant Internet traffic generators. (R. Rodrigues). Different measurement studies indicate that P2P traffic 1389-1286/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comnet.2011.10.014 R. Lopes Pereira et al. / Computer Networks 56 (2012) 642–660 643 represents from 35% [3] to 70% [4,5,1] of the traffic carried the performance of HyperText Transfer Protocol (HTTP) by Internet Service Providers (ISPs). and how ASR may alleviate it. Finally, a migration path ISPs are left at a crossroad. On the one hand, they are for the introduction of ASR is proposed and evaluated. Sec- confronted with the performance expectations of most tion 7 presents our experience of using an ASR enabled customers (who do not use P2P and suffer a performance eMule client to download files from live Internet swarms. penalty on their applications), with the bandwidth costs Section 8 discusses some of the open issues of ASR and of supporting P2P traffic and with the potential legal issues the obstacles and opportunities for the introduction of of P2P, as some of the shared files infringe copyrights. On ASR on the Internet. We conclude this paper and present the other hand, with the revenue potential of clients that our plans for future work in Section 9. use P2P and the pressure to maintain network neutrality. A compromise must be reached between the needs of ISPs, 2. Related work who want to use their networks to provide service to all their customers while minimising their investment, and In the past few years, as the growth of P2P FS traffic be- P2P users, who expect to be able to use P2P applications came obvious, researchers began working towards solutions with reasonable performance. to the problem of containing this traffic, resorting to traffic Current P2P applications do not consider network locality. These solutions may be divided into three ap- topology when selecting peers with whom to share files. proaches: the ones where servers/tracker or peers rely on For instance, a peer may download a file from another peer data from the network provided by its operators (ISPs) in outside its ISP when that file might be available from peers the form of oracles; the ones that concentrate the locality ef- within the same ISP, incurring higher cost for the ISP. How- forts on the server/tracker which, by itself, gathers together ever, several studies have shown that P2P FS swarms show close-by peers; and the ones which follow the end-to-end locality properties which are not exploited by current P2P architecture of the Internet, having the peers themselves file sharing protocols [6–9]. ISPs will benefit from the use gather information and determine the best peers to use. of a peer selection algorithm which takes into account topology information to guide peer selection, favouring local peers. 2.1. ISP supported solutions In this paper we present Adaptive Search Radius (ASR), a peer selection algorithm that allows popular P2P FS BNS is a locality-aware method for reducing inter-ISP applications, such as eDonkey/eMule (herein referred to P2P traffic that takes network topology into account [13]. simply as eMule), BitTorrent or Gnutella, to generate less This method, which is applied to BitTorrent, may either Internet traffic, while improving the download duration be implemented by the tracker with collaboration from perceived by the users. This algorithm limits the distance, the peers or by the ISPs. When implemented on the tracker, in network hops, of the peers a node may download from, peers indicate their ISP, so that the tracker may group them promoting the creation of small, local clusters of peers together. When implemented by the ISP, a transparent exchanging a file, instead of large worldwide networks of redirector is used, which, using Deep Packet Inspection, peers. Each peer still receives the same amount of traffic intercepts the communication from peers inside an ISP’s (it must receive a full copy of each downloaded file), but network with the tracker. The sources (peers) provided each data packet travels fewer network hops, and conse- by the tracker are filtered and replaced in order to force quently consumes bandwidth over fewer links [10,11]. the peer to communicate mainly with other peers within By fetching data from closer (in network hops) peers, the ISP’s network. In order to provide worldwide benefits, fewer data will cross inter-ISP links, namely expensive every ISP would have to install it or every peer and tracker international inter-ISP links. This is expected to greatly would have to be upgraded. reduce ISPs’ expenses with peering traffic. Peers within ISPs have a unique knowledge of their networks which the same ISP should be preferred over peers on other ISPs, can never be discovered by a client, including information allowing for a better utilisation of the less expensive intra- regarding the financial cost associated with each link, peer- ISP links. The freed bandwidth on the inter-ISP links will be ing agreements, routing policies and current utilisation of available for other applications, which may benefit from links. Being that the reduction of the costs supported by better performance by not having to compete with as the ISPs is the central objective of P2P localisation solu- much P2P traffic. tions, it makes sense to use this information. As such, ora- ASR is a low overhead, easily implementable solution cle proposals have emerged, where ISPs provide clients which does not require knowledge of the underlying Inter- with interfaces for discovering the ISPs preferences for net topology. Using cross-layer information, ASR may be routing P2P traffic or for sorting possible peers. P2P clients implemented with zero overhead. It may be deployed by consult the oracle in order to select which peers, among gradually upgrading the existing P2P client software [12]. the ones it knows, should be used [14–16]. Benefits are This paper is organised as follows. Section 2 reviews even greater when ISPs cooperate among themselves. and discusses related work. A background on eMule is The IETF has formed a working group entitled Applica- 1 provided in Section 3.